1. Field of the Invention
This invention relates generally to a system and method for controlling an ultrasonic welding process and, more particularly, to a system and method for multi-mode control of an ultrasonic welding process, where the modes include weld energy, weld time and part compression displacement.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combine a battery and a main power source, such as an internal combustion engine, fuel cell system, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. The batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead acid, etc. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
A modern electric vehicle battery pack typically includes a plurality of battery modules, where each module includes a plurality of battery cells, and where the number of modules and cells determines the battery pack voltage. In one particular battery pack design, common polarity electrical tabs of three battery cells (tri-pack) are welded together and to an interconnect bus bar. Several of the tri-pack of battery cells are welded to the same interconnect bus bar within a single cell module, so that the cells in each tri-pack of battery cells are electrically coupled in parallel and the several tri-pack of battery cells are electrically connected in series within the module. A group of modules is electrically connected in parallel or series to form a battery section and a number of battery sections are electrically coupled together in a single housing to provide a battery pack.
Ultrasonic or vibrational welding using ultrasonic frequencies is known in the art. A horn or sonotrode of a vibrational welding machine is placed against the parts being welding and ultrasonic sound energy from a transducer causes the parts to vibrate, which creates friction that generates a large amount of heat to bond the parts together. In one specific application, vibrational welding is used to bond the common positive and common negative tabs of the tri-pack of battery cells to the interconnect bus bar referred to above.
The welding control strategy for vibrational welding for at least this application uses a single mode control from the several control parameters that are available. For example, the known single mode control for this process generally only monitors and controls one of welding energy, welding time or compression displacement distance of the tabs being welded relative to the interconnect bus bar. Particularly, the control strategy of the welding process may control or terminate the welding process based on only the amount of energy provided at the weld, which increases through time based on a power input to the ultrasonic horn. Alternately, the welding control strategy may control the welding process only by the duration of the welding time. Or, the welding process may be controlled by only the compression displacement of the parts being welded during the welding process.
Each one of these single mode welding control strategies has proven to be an applicable technique for providing the desired weld integrity for a particular application. However, using single mode welding control has been shown to cause random low probability weld integrity issues.